Chromate conversion coatings are superficial layers which are formed at the surface of a base material, such as zinc or cadmium, to shield the base material or metal from corrosive interaction with the environment. For instance, it is well known that zinc or zinc plated surfaces develop a white, powdery corrosion product which is referred to as "white rust." The formation of such corrosion product is effectively inhibited by chromate conversion coatings.
Chromate conversion coatings are formed, for example, on zinc or zinc plated surfaces in a chemical reaction of the surface metal with a chromating solution. The chromating solution typically contains chromic acid and chromates or dichromates and may contain nitric and sulfuric acids, wetting agents and other additives. A number of chromating baths are commercially available, and as it is true for most proprietary processing baths, the precise analysis of such baths is typically not available. While chromating bath constituents may be purchased as soluble solids, a liquid bath concentrate is preferred. In preparing the bath, the concentrate is diluted with water to the desired concentration.
In the surface treatment of articles, such as relay cans of a draw quality, cold rolled steel, the articles are, for example, zinc plated to a preferred thickness of 0.005 mm. After typical rinsing in overflowing cold water and a brief drain period, the articles are immersed into a typical chromating bath. In forming the chromate conversion coating, a chemical reaction between the metal surface and hexavalent chromium in the chromating bath oxidizes the base metal, and the hexavalent chromium becomes trivalent. Compounds of the trivalent chromium precipitate at the oxidized metal surface as a gel, entrapping some of the hexavalent chromium from the solution.
According to known procedures, the chromated articles are rinsed in water and then dried. The rinsing steps may consist of a first, cold water rinse, followed by a second, warm water rinse, whereby the warm water temperature is preferably kept in the range of 50.degree. C. The chromate conversion coatings are temperature sensitive and, in view of such a sensitivity, a hot water rinse would normally not be used. However, the chromate coatings are also initially soft with a low scratch resistance until the coatings have dried. Thus, a warm water rinse is a compromise to promote drying after rinsing without running the risk of damaging the newly formed coatings.
A problem occurs frequently in commercial chromating operations in that chromate coatings are rinsed off or become otherwise damaged during rinsing operations subsequent to the chromating step and prior to drying. The occurrence of such damage is of concern since the protection of the base metal is partially removed, and the appearance of the chromated surfaces becomes spotty. The apparent flaking of the chromate coatings have occurred in spite of careful handling of chromated parts during the rinsing steps.
In a chromating process, a high incidence of articles having such a flaked and spotty appearance causes rejections and tends to slow down and increase the cost of the chromating process. Consequently, a need exists to reduce or eliminate the occurrence of peeling or damage of the newly formed chromate coatings during the time between the removal of parts from the chromating bath until the coatings are dry.